CW laser amplifier

ABSTRACT

A solid state laser amplifier includes a diode pump source and a crystal capable of lasing light at a fundamental wavelength. The source is directed into the crystal, and a seed beam is also coupled along an input path into the crystal to initiate lasing, which amplifies the seed beam. The amplifier is intended for intermittent use, or use over a wide range of input power levels, so that variations of the energy input and output give rises to extreme thermal challenges. To manage heat stress in the amplifier crystal, both the pump beam and the seed beam are directed into the crystal through a cap of undoped material which is non-absorbent at the seed or pump wavelength, and is diffusion bonded to the crystal, thus serving as a thermal reservoir and allowing the laser to operate over a broad power band, with high gain over an extended range of saturation values, and to maintain stable operation during intermittent or changing operation of the seed laser. In a preferred embodiment the crystal is pumped from both ends by separate diode sources, with a continuous wave pump power of 10-40 W or more, and utilizes a vanadate crystal which lases at the band of a garnet or vanadate laser, providing a gain of two or more over an input seed beam power range from milliwatts to ten watts or more.

The present application is a continuation of allowed U.S. applicationSer. No. 09/071,068, filed on May 1, 1998. The contents of theaforementioned application is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to laser amplifiers, and in particular toa solid state continuous wave laser amplifier.

SUMMARY OF THE INVENTION

The present invention is a solid state laser amplifier. The amplifierincludes a laser crystal which is capable of being pumped to producelaser light at a fundamental wavelength, and a diode pump source whichis directed into the laser crystal. A coupler receives a seed beam anddirects it into the crystal along an input path to initiate lasing whichamplifies the seed beam. Both the pump beam and the seed beam aredirected into the crystal through a cap of material which is diffusionbonded to the crystal and which is undoped, and does not absorb energyat the pump wavelength. The cap serves as a thermal reservoir andreduces heat stress of the laser crystal during varying cycles ofoperation. This allows the laser to operate over a broad power band,with high gain over an extended range of saturation values, and maintainstable operation during intermittent operation of the seed laser. Byalleviating thermal excursions, it also allows more stable operationwith a crystal having small dimension, higher doping concentration, orboth.

In a preferred embodiment the crystal is pumped from both ends byseparate diode sources, with a continuous wave pump power of ten toforty watts or more, and the amplifier utilizes a vanadate crystal whichlases at the band of a garnet (e.g. YAG) or YVO₄ laser. The system soconfigured may provide a gain of two or more over an input seed beampower range from milliwatts to ten watts or more. The amplifier mayoperate as a single-pass amplifier, with direct output of the amplifiedbeam, or it may employ a return mirror which reflects the amplified beamback into the crystal to provide increased gain. When used with a returnmirror, the output beam may be separated from the path, e.g., blockedfrom reaching the input laser, by a polarization coupling assembly, suchas a Faraday rotator and polarization beam splitter.

Preferably the pump source utilizes a packaged diode array havingbundled diode outputs that provide a high power but relatively poorquality beam, with a power in the range of two to thirty watts. Thisbeam is directed by one or more relay lenses into the crystal, and theseed beam is coupled centrally along the input path, inside the pumpmode volume, so that an amplified beam of higher quality is produced.The amplifier is a simple and effective unit to upgrade an existingpiece of equipment, such as a lower power or a prior generation YAGlaser or medical laser console, of the type common for an ophthalmic ordermatologic clinical laser system This essentially multiplies itsavailable power while allowing the console to operate with its existingpulse selection, power stabilization, and sequencing control circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other desirable features will be understood from thediscussion below of representative embodiments and principles ofoperation, together with the drawings of several illustrative systemsand details thereof, wherein.

FIG. 1 shows one embodiment of the invention;

FIGS. 2 and 2A show representative heat distributions of the prior artand of the invention;

FIG. 3 illustrates another embodiment of the invention;

FIG. 3A illustrates a multi-pass embodiment of the invention; and

FIG. 4 illustrates details of the crystal in systems of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a basic embodiment of the invention wherein a laser crystal10 is set up to be pumped by a diode pump source 12 of which the outputbeam 13 is focused or relayed by a relay lens 15 into the crystal. Aturning mirror M1, which is transmissive at the pump wavelength and hasa lower side that reflects the fundamental wavelength, is positioned infront of the crystal 10 and receives an input or seed beam 20 at thefundamental wavelength from a laser source 22 which may, for example, bea pulsed laser of the type commonly employed for clinical treatmentprocedures. The crystal 10 may, for example, be a vanadate crystal whichlases at about 0.1 micrometer, while the diode pump source may be an 808nanometer source operating at about ten to thirty watts power level.Opposed sides of the crystal 10 are heat sunk by heat sink 24. The pumpbeam is ON continuously, and produces a high population of excitedstates in the crystal so the crystal lases when the seed beam isintroduced and produces an amplified output beam 26. Thus, in thisembodiment, the device operates as a single-pass amplifier in which thediode source provides an excited population and the seed beam achievesthe threshold necessary for lasing activity. The vanadate crystal may bea small crystal, for example, a block 3×3×4 millimeters. In accordancewith a pricipal aspect of the invention, the crystal 10 has an end cap10a on its pumped end.

The end cap 10a is formed of essentially the same material as the lasercrystal itself but is undoped, or doped to a much lower level, so thatvery little or none of the 808 nm pump light is absorbed in the end capitself. The end cap is diffusion-bonded to the rest of the crystal. Thisis done by polishing both the end cap and the crystal end opticallysmooth and flat and placing the cap onto the end of the crystal,preferably also applying at least one of an elevated pressure or heatfor a sufficient time to create a unified junction between the twopieces. This provides a single crystal body, which is free of extraneousinternal reflective of diffractive boundaries or artifacts, yet in whichthe heat of absorption is generated substantially only in the doped,absorbing portion of the crystal away from the end, while in otherrespects the entire body of the crystal acts as a homogeneous crystal.As a result, the mechanism of heat generation, which is localized, istempered by conductive cooling from a portion which extends furtheroutside of the heated region.

FIG. 2 illustrates a representative shape of the heat distribution overthe cross-section of a laser crystal which would occur during diodepumping. The pump light is directed centrally into the crystal, and heatbuilds up near the center while the edges remain cooled by the heatsink, such as sink 24. In typical operation, when the seed laser isturned on, its power is amplified, for an example, by a factor of abouttwo, and energy is coupled out of the crystal.

When the seed laser has a relatively high power level, such as a tenwatt pulse, then the amplification couples a comparable amount ofenergy, which had been absorbed from the pump light, out of the crystal,and the heat economy of the crystal is relatively manageable. On theother hand, when the seed beam is of low duty cycle or power, such as a100 mW level, or when the seed laser is turned OFF, then substantiallyall of the pump energy ccontinues to be absorbed in the crystal withoutany quick energy loss mechanism being active, and the crystaltemperature starts to climb quite rapidly. As shown in FIG. 2, underthese circumstances the temperature in the center of the crystal mayrise to several hundred degrees higher than at the edges.

FIG. 2A is a view similar to FIG. 2 showing the form of resultingtemperature distribution under zero or low-input conditions of thecrystal in accordance with the present invention, having an end cap 10apositioned on the crystal 10. Since the material of crystal end 10a doesnot absorb pump radiation, hence does not heat up internally, thismaterial operates as a passive thermal reservoir to conduct heat out ofthe crystal 10 across the plane of the actively doped end portion of thecrystal. This maintains the interior temperature of the lasing portionof the crystal at a level substantially below that of the bare(uncapped) crystal (FIG. 2), and flattens the thermal gradient acrossthe thickness dimension of the crystal in the region where the highestpump absorption occurs. The effect is quite pronounced since the crystal10 is preferably highly doped, e.g., doped to a level sufficient toabsorb the pump light in a single pass, and therefore has a relativelyshort extinction path length.

FIG. 3 illustrates a presently preferred embodiment of a laser amplifier100 in accordance with the present invention. Laser 100 in thisembodiment employs a laser crystal 30 which is pumped by two diodesources 40a and 40b, one pumping each end of the crystal. Each of thesources 40a and 40b may, for example, have an output power ofapproximately twenty or thirty watts at 808 nanometers, so thatsubstantial pump power is placed into the crystal. As before, an inputlaser 50 projects its beam 51 along an input path into the crystal 30.In the illustrated embodiment, beam 51 is coupled by a first foldingmirror M1, which is completely transmissive at the pump wave length andcompletely reflective at the input (seed) wave length. At the other endof the crystal, a similar mirror M2 having its upper surface refectiveat the seed wavelength, couples the amplified beam out of the system asan output beam 52. Just as for the basic embodiment described above, thedoubling crystal 30 has an end cap 31a on its input end at the rightside) and an end cap 31b on the left end of the crystal which is pumpedby source 40b, and these end caps are formed of a non-pump absorbing,undoped material, and are diffusion-bonded to the respective crystalfaces.

In many respects, this architecture and layout are similar to the diodepumped lasers described in Applicants' U.S. patent application5,663,979, and particularly to the systems shown in FIGS. 16-21 thereof,wherein diode array packages pump a crystal from one or both ends viarelay lenses which allow a placement of one or more turning mirrors forproviding return, input, or output paths for the active components. Inaddition, this construction advantageously utilizes the heat sinkingarrangements described in commonly-owned U.S. patent application Ser.No. 08/865,508 filed May 30, 1997, wherein the heat sinks are operated,optionally with dynamic control keyed to the prior and/or nextactivation, to maintain an isotherm in the mode volume. The fulldisclosures of the foregoing patents and the foregoing patentapplication are hereby incorporated herein by reference.

Returning now to a discussion of FIG. 3, the figure also shows end caps31a, 31b on the crystal 30, which as noted above are non-absorbing atthe pump wavelength, and are diffusion-bonded (sometimes referred to asoptically-contact bonded) to the crystal 30 to serve as passive thermalreservoirs to supplement the conductive cooling pathways of the laserwithout complicating its heat generation, and thus prevent extremetemperature excursions of the crystal. As noted above, preferably thecrystal 30 has heat sinks at opposed sides, e.g., the top and bottom ofa rectangular brick, or circumferentially surrounding a rod, and theseheat sinks may advantageously extend similarly over the sides of the endcaps. In the latter case, the supplemental cooling is actively increasedwith the heat sinks, while subject to a smoothing delay due toconduction through the caps.

While the amplifier 100 of FIG. 3 operates in a single-pass from theinput 51 to the output 52, the invention also contemplates a multipassconstruction. In one such construction, a return mirror is provided toreflect beam 52 back through the crystal, thereby increasing theefficiency of the amplification process. Such a system is indicated inFIG. 3A. In this system, the first-pass amplified beam 52 passes throughthe crystal 30, and is directed to a mirror M3 which reflects the beamback into the crystal where it undergoes further amplification and isdirected out along the input path 51 as an enhanced amplified beam 52b.A Faraday rotation isolator 60 is placed in the path, which allows theinput beam 51 to enter the amplifier while blocking all but a definedpolarization of the return beam 52b. Beam 52b is coupled out from thepath by a polarization beam splitter 64 placed ahead of the source, thusseparating the amplified output beam 52b along a distinct output path.With this construction, using diode arrays or packages 40a, 40b, eachhaving a power of about twenty or thirty Watts, the laser crystal iscapable of amplifying input seed beams over a broad range of powerlevels with fairly uniform gain. Thus, for example, a 100 milliwatt seedbeam is amplified to a 200 milliwatt output beam, while a 10 or 20 wattseed beam may be amplified by a factor of 1.5-2, the gain being slightlylower at higher input powers due to the increased saturation of theamplifier. As noted above however, when the seed beam is of low power,then a correspondingly greater portion of the pump beam is beingabsorbed in the crystal and the energy is not removed in the form ofamplified output. For this reason, the presence of the end caps becomescritical for avoiding sharp fluctuations in the temperature of thecrystal, or degradation of the beam quality or amplifier gain.

FIG. 4 further illustrates a preferred arrangement of the seed and pumpbeams in the amplifier of any of the foregoing figures. As shown in FIG.4, the cross-section of the crystal 30 as viewed from the end iscentrally pumped from the diode source with a pumped mode volume P. Theinput mirror M1 directs the input beam 51 centrally within the area P sothat the amplified beam occupies a smaller volume, resulting in acleaner output. Similarly, the mirror M3 of FIG. 3A may, for example, bea focusing or slightly concave return mirror which focuses the outputbeam centrally within the corresponding pumped region at the other endof the crystal. As will be readily understood, it is also not necessaryto utilize both mirrors M2, M3 in the two-pass embodiment of FIG. 3A;these may be replaced by a single mirror M4 (not shown) placed acrossthe beam axis.

In all of the foregoing constructions, the crystal 30 is capable oflasing at the wave length of the input source 50 so that for exampleboth may be YAG laser crystals. However, preferably, the crystal 30 is ayttrium vanadate crystal which has a broad lasing band which operateseffectively with either YAG or vanadate laser sources. In practicalembodiments of the system, all elements except for the source 50 may beassembled as a single unit having an input port which may, for example,accept a standard type of optical fiber connector, and having an outputport for delivering the beam 52b, as well as a simple power lineconnection for powering the pump source, and possibly coolingarrangements, if an external cooler or plumbing is to be used. In thiscase, the input beam 51 provided by separate input laser 50 may simplyconnect to a common input port of the assembly so that the assembly isreadily adapted for working with any existing pulsed or continuousoperation laser system of the appropriate wave length.

The foregoing illustrations of amplifying architecture are intended asillustrative of the principle of the invention. However, the inventionin its broadest aspect, using a laser crystal having uniform thermaltransfer and structural/mechanical characteristics, but a stepped dopingprofile, will find may uses in lasers, amplifiers and doublers, allowingcoupling between optically active elements that reduces the constraintson associated doping levels, coupling parameters, and requisite thermalcontrol systems. The invention being thus disclosed, variations andmodifications will occur to those skilled in the art, and all suchvariations and modifications are considered to lie within the scope ofthe invention as defined by the claims appended hereto.

What is claimed is:
 1. A continuous wave laser amplifier, comprisingapump source for emitting laser light at a pump wavelength λ_(p), acrystal placed in a laser cavity and configured to emit laser light atan output wavelength λ_(o), said laser light of said pump source beingdirected into the crystal, at least one optical element position tocouple a laser seed beam along an input path into the crystal so thatthe crystal amplifies power of the laser seed beam into an amplifiedbeam at said output wavelength λ_(p), an output coupler for coupling atleast a portion of said amplified beam out of the cavity therebyremoving energy, and wherein said crystal includes an end of non-lasingmaterial forming a passive thermal reservoir to stabilize operation whenoutput power is low, thereby preventing overheating and crystal damage.2. A continuous wave laser amplifier according to claim 1 wherein thepump source is a diode source which includes a first laser diode arrayarranged to pump a first end of the crystal, and a second laser diodearray arranged to pump a second end of the crystal, and wherein each ofsaid first and said second ends are formed of non-lasing material.
 3. Acontinuous wave laser amplifier according to claim 1 wherein saidnon-lasing material is diffusion-bonded to said crystal.
 4. A continuouswave laser amplifier according to claim 3, wherein said pump sourcepumps a mode volume, and said seed beam is coupled into a subregion ofthe mode volume.
 5. A continuous wave laser amplifier according to claim3, wherein said crystal is a vanadate crystal.
 6. A continuous wavelaser amplifier according to claim 2, wherein the first and second endshave end caps formed of undoped crystal material which are diffusionbonded to a central block of doped crystal material to form saidcrystal, and said crystal amplifies a 1.06μ seed beam.
 7. A continuouswave laser amplifier according to claim 6, wherein said amplified beamis output coupled along an output beam path by oblique reflection from amirror.
 8. A continuous wave laser amplifier according to claim 6,further comprising a cooling member to cool the crystal and the endcaps.
 9. A continuous wave laser amplifier according to claim 1, furthercomprising a return mirror for directing said amplified beam back thoughthe crystal to form a multipass amplified beam, and further comprisingan output selector for blocking the multipass amplified beam from theinput path and selectively directing it to an output path.
 10. Acontinuous wave laser amplifier according to claim 9, wherein the outputselector comprises a Faraday rotator and a polarization beam splitter.11. A continuous wave laser amplifier, comprisinga pump source foremitting laser light at a pump wavelength λ_(p), a crystal having an endformed of non-lasing material positioned to receive said laser light ofsaid pump source, said crystal being configured to emit laser light atan output wavelength λ_(o), said non-lasing material forming a passivethermal reservoir to stabilize operation of the amplifier when outputpower is low, and a laser seed beam coupled along an input path into thecrystal so that the crystal amplifies power of the laser seed beam intoan amplified beam at said output wavelength λ_(o).